As revealed by Parker et al in Rubber Chem. & Tech., Vol. 65, 245 (1992), NBR latexes that are converted to hydrogenated NBR latexes by the method disclosed in U.S. Pat. No. 4,452,950 are prone to an undefined crosslinking side reaction that occurs concurrently with the desired reduction of double bonds. This crosslinking reaction produces "gelled" or "crosslinked" saturated hydrogenated NBR latex particles. For many latex applications, this crosslinking can have a beneficial effect. For instance, latex cast films from such a material may form continuous rubber coatings with good tensile, elongation and elastic recovery properties. Unfortunately, however, when highly crosslinked latexes are coagulated by common techniques known in the art, the resulting dry rubber mass is unprocessable and unable to flow to any significant extent because of its macroscopic three dimensional crosslinked structure. The material essentially has an "infinite" molecular weight in this form and cannot be processed by conventional rubber equipment.
One possible solution to this dilemma was revealed in U.S. Pat. No. 5,039,737 whereby the crosslinked "hydrogenated" NBR latex prepared by U.S. Pat. No. 4,452,950 is first treated with ozone to cleave residual unreduced double bonds. This treatment resulted in lowering the molecular weight of the rubber with concurrent generation of both terminal aldehyde and carboxylic acid end groups at the cleavage sites. Unfortunately, although the originally crosslinked hydrogenated NBR rubber can be made soluble in a good solvent for hydrogenated NBR (e.g. chloroform) if immediately coagulated from the latex and redissolved, upon drying, the soluble rubber recrosslinks again to become useless. This problem could be overcome, however, as revealed in U.S. Pat. No. 5,039,737 by reducing the terminal aldehyde groups in the polymer using the strong and relatively expensive reducing agent . . . sodium borohydride, in ethanol solution. Presumably, the aldehyde groups are converted to terminal polymeric alcohol groups (after hydrolysis of the borate intermediates) that are not prone to recrosslinking since the resulting polymer is reported to remain soluble. Unfortunately, this method of using sodium borohydride to obtain a soluble processable hydrogenated NBR rubber is cumbersome, expensive, uses alcohol solvents and evolves hazardous hydrogen gas during the process.
In contrast to the process described in U.S. Pat. No. 4,452,950 and in Rubber Chem. & Tech., Vol. 65, 245 (1992), commercial hydrogenated NBR dry rubber is prepared by a completely different technique. In this method, dry NBR rubber is first ground into particles and then dissolved in a solvent. To the resulting cement is then added a noble metal catalyst. The mixture is then subjected to hydrogen pressure at elevated temperatures to effect reduction of the double bonds. Solvent and the expensive catalyst are then removed in a series of steps resulting in hydrogenated NBR rubber that has essentially the same molecular weight and structure as the original NBR. Therefore, if the original NBR was processable, the resulting hydrogenated NBR most likely will be processable as well. Whereas this method will easily produce processable hydrogenated NBR, it suffers from being an extremely costly and complicated process. Hazardous hydrogen gas is used and solvents and valuable metal catalysts are unable to be fully recovered.